Field of the Invention
The present invention relates to a solid-state imaging device and an imaging system.
Description of Related Art
In recent years, imaging systems such as video cameras and electronic still cameras have been generally and broadly propagated. Charge coupled device (CCD) solid-state imaging devices or complementary metal oxide semiconductor (CMOS) solid-state imaging devices are mounted in such imaging systems. In such solid-state imaging devices, a plurality of pixels are disposed in a 2-dimensional matrix, signal charges generated by photoelectric conversion units such as photodiodes installed in the pixels on which light is incident are held in charge retention units, signal charges are amplified in amplification units installed in the pixels, and the amplified signal charges are output as pixel signals. At this time, in general CMOS solid-state imaging devices, pixel signals from pixels arrayed in a 2-dimensional matrix are sequentially read for each column in the related art.
While CCD solid-state imaging devices are manufactured through dedicated manufacturing processes, CMOS solid-state imaging devices can be manufactured using general semiconductor manufacturing processes. Accordingly, in CMOS solid-state imaging devices, for example, multiple functions can be realized by embedding various functional circuits in the solid-state imaging devices as in system on chips (SOC).
Therefore, examples in which solid-state imaging devices including analog digital conversion circuits (hereinafter referred to as “A/D conversion circuits”) are used as CMOS solid-state imaging devices (hereinafter referred to as “solid-state imaging devices”) mounted on imaging systems have increased. As an A/D conversion circuit included in such a solid-state imaging device, for example, there is a slope integration type A/D conversion circuit (see Japanese Unexamined Patent Application, First Publication No. 2008-118634) that performs analog/digital conversion using a ramp wave.
In the slope integration type A/D conversion circuit, a magnitude relation between a ramp wave (reference voltage) gradually varying at a predetermined slope over time and a pixel signal (analog signal) output from each pixel of the solid-state imaging device is compared, and a time from start of the comparison to reversion of the magnitude relation between the ramp wave and the pixel signal is counted by a counter. Here, a counted value (count value) counted by the counter is set to a value (digital value) indicating the magnitude of a pixel signal, that is, an analog signal output from the pixel.
As in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2008-118634, a ratio of the entire power consumption of the solid-state imaging device occupied by power consumption of an A/D conversion circuit is large in the solid-state imaging device including the A/D conversion circuit. In an A/D conversion circuit performing analog/digital conversion using a counter as in a slope integration type A/D conversion circuit, power consumption of a counter which is performing analog/digital conversion has a large ratio. Therefore, in a solid-state imaging device of a column A/D scheme including a plurality of A/D conversion circuits at intervals of one column or a plurality of columns of a plurality of pixels disposed in a 2-dimensional matrix, power consumption of a counter in the A/D conversion circuit occupies a considerable ratio of the entire power consumption of the solid-state imaging device.
With recent requests for numerous pixels and high quality in solid-state imaging devices, the number of A/D conversion circuits included in a solid-state imaging device or the number of bits of digital values to be output has increased. For example, A/D conversion circuits of 12 bits are included for each column of a plurality of pixels disposed in a 2-dimensional matrix in a solid-state imaging device. Therefore, power consumption of counters in A/D conversion circuits included in a solid-state imaging device has a larger ratio, and thus power consumption of the solid-state imaging device increases. Further, with requests for high-speed operations in imaging systems, it is also necessary for a solid-state imaging device to operate at a high speed, and thus power consumption of counters in A/D conversion circuits included in solid-state imaging devices has further increased.